When utilizing an electronic instrument, including but not limited to electric guitar, varying amounts of harmonic distortion are sometimes desired. Thus, a system that employs adjustable distortion is preferred. In many applications, the harmonic distortion characteristics of tube amplifiers manufactured in the 1950s and the 1960s are considered to be definitive in amplifier design. A number of factors have contributed to the superiority of these tube amplifiers.
Although the amplifiers were not designed to be overdriven, they had to have enough reserve gain to allow for weak guitar pickups. Thus, when a stronger pickup is used, the volume can be increased until tubes, transformers and power supplies are overdriven. Output stages are usually push-pull followed by an impedance-matching transformer which connects to the speaker(s). When gain reaches a level where the tubes are overdriven, the output transformer will begin to go into saturation, creating decreasing even- and odd-order harmonics.
In designing amplifiers presently, modern designers are careful to use matched tubes in the push-pull output stage. In 1950s and the 1960s, tubes were more randomly selected and therefore, unmatched tubes were installed, which possibly contributed to some nice sounding second order harmonic distortion from the unbalanced output transformer.
In a traditional amplifier, in order to get the desired output distortion characteristic, the amplifier must be set to a specific volume. For example, on a given amplifier, a volume setting of “5” might be too “clean” sounding, “8” would be too distorted, and “7” would represent a middle ground. But although the volume of “7” may produce the desired distortion characteristic, this volume would not be appropriate for all performance situations.
A number of approaches to distortion devices have been attempted. In the 1970s, amplifier designers would add an extra stage of pre-amplification, which would allow a pre-amplification tube to be heavily overdriven. This design necessitated a “Drive” pot to control the amount of distortion followed by a “Master Volume” pot to control the signal level to the power amp. In order to get the desired output-transformer distortion, the Master Volume has to be increased to very high levels, which is not always desirable depending on the performance.
Previous approaches included power attenuators, which are connected between the amplifier output and the speaker. The power attenuators allowed the amplifier to be run at a high enough volume to get the desired distortion, while reducing power to the speaker; any extra power is dissipated as heat. This system is problematic because it in expensive, it must be carried, hooked up, the heavy tube amplifier is still required, and a second amplifier and speaker are required for the optional post-distortion effects.
An adjustable-efficiency speaker has also been used for distortion, but it still requires the heavy, expensive tube amp. The efficiency of the speaker can then be decreased, the excess power again being dissipated as heat.
Selectable output power in an amplifier has allowed the user to select the number of output power tubes in service and/or selecting operation in class-A or class-AB. But this approach assumes that the desired distortion characteristic comes only from power tubes, usually pentodes or beam power tubes and thus, only in the highest power configuration would the transformer saturate, contributing its unique distortion characteristics.
A solid-state transconductance power amplifier such as Marshall's ValveState™ amplifier can be used for distortion, but this approach assumes the transconductance characteristics of tubes are the only source of the desired distortion. No output transformer is used.
Digital solutions have also been employed to create desired distortion. Many digital effects manufacturers use an approach where the output sounds from a model amplifier are sampled with a microphone(s), various signals are input to it, and the output is analyzed and digitally simulated.
Analog circuits have been designed to simulate some of the characteristics of transformer saturation.
Finally, there have been experiments with adjustable crossover distortion, but because crossover distortion in class-AB amplifiers is not always desirable, this solution has not been commercialized.